Making a shape key with a value that is the average of other shape keys
The data_path on a driver variable is relative to the ID object you select, in your case objects "Template".  The location x path from here would be "location.x" or "location[0]" for example.

Select key as an ID object instead and your keyset.  An example path to "Key 1" key's value is demonstrated in image.



Alternatively from the mesh  bpy.context.object.data.shape_keys which returns a pointer to a bpy.data.shape_keys keyblock.



Screenshot of average shapekey driver setup



PS.  The panel on RHS is a shameless plug for my Driver Manager Addon.